In recent years, power storage devices such as lithium ion secondary batteries have been developed.
Examples of such power storage devices include a power storage device having an electrode formed using lithium iron phosphate (LiFePO4), which is a composite oxide, as an active material. The power storage device having an electrode formed using LiFePO4 has high thermal stability and favorable cycle characteristics.
As an example of a method for forming a composite oxide such as LiFePO4, a hydrothermal method can be used (e.g., Patent Document 1). A hydrothermal method is a method, which is performed in the presence of hot water, for synthesizing a compound or growing crystal of a compound.
By using a hydrothermal method, even a material which is less likely to be dissolved in water at ordinary temperatures and pressures can be dissolved, and thus a substance which is hardly obtained by a production method performed at ordinary temperatures and pressures can be synthesized or crystal growth of such a substance can be conducted. Further, by using a hydrothermal method, microparticles of single crystals of an objective substance can be easily synthesized.
Using a hydrothermal method enables to be a desired compound, for example, in the following manner: a solution containing a raw material is introduced into a container resistant to pressure and be subjected to heat and pressure treatment; and the treated solution is filtered.